The present invention relates to an image reading apparatus for reading an image of a document placed on a document table, and performing shading correction on the read image on the basis of a reference signal obtained from a white reference plate.
In general, digital copying machines, for example, comprise a scanner section for reading an image, and a print section for printing the image read by the scanner section.
The scanner section uses a CCD line sensor for reading an image. The level of voltage accumulated in the CCD line sensor are significantly influenced by variations in the illuminance of an exposure lamp, a reduction in the illuminance with time, and/or level variations in the pixels of the CCD line sensor. In light of this, the white and black levels are set immediately before image reading, so that the same image can be copied regardless of conditions (shading correction).
To this end, a white shading correction plate (hereinafter referred to as a “white reference plate”) is used, and a signal from the white reference plate is read by a scanner section with several lines of elements, thereby correcting variations in the illuminance of an image, using the average level of the read signal as a white reference level.
When ripples occur in the power supply line of the scanner section, they also occur in image signal lines, thereby causing oblique lines or uneven density to be generated in a resultant image. In the prior art, to suppress the ripples, an electrolytic capacitor is added to the power supply line to increase the capacity of the power supply line.
However, this inevitably increases the capacitance of the capacitor, and involves disadvantages, such as an increase in substrate size and/or cost, or the disadvantage that a package component cannot be used.
Degraded images having density or color differences result from the ripples that occur in image signal lines. Since the occurrence of ripples in the voltage of the power supply is asynchronous with the generation of image data, the ripples do not occur at corresponding positions between the lines.